As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, and a multi-task machine having plural such functions, a known ink jet recording apparatus of a liquid ejection recording type uses a recording head that ejects, for example, ink liquid droplets. The image forming apparatus of this type ejects ink droplets onto a sheet during conveyance from the recording head to perform image formation (used synonymously with recording, printing, and imaging). Examples of the image forming apparatus include a serial-type image forming apparatus in which a recording head ejects liquid droplets to form an image while moving in a main scanning direction and a line-type image forming apparatus using a line-type head in which a recording head ejects liquid droplets to form an image without moving.
Note that in the present invention, an “image forming apparatus” refers to an apparatus (including a simple liquid ejection apparatus) that ejects ink droplets onto a medium such as paper, a thread, a fiber, a fabric, leather, metal, a plastic, glass, wood, and a ceramic so as to perform image formation. Furthermore, “image formation” refers to forming on a medium not only relevant images such as characters and graphics, but also irrelevant images such as random patterns (i.e., an apparatus called a liquid droplet ejection apparatus or a liquid ejection apparatus that merely shoots liquid droplets on a medium). Furthermore, “ink” is not limited to one as generally called ink, but it is used as a generic name of various liquid available for the image formation, such as recording liquid, fixing treatment liquid, a DNA sample, and a patterning material. Furthermore, the material of a “sheet” is not limited to a piece of paper. That is, the sheet refers to ones including an OHP sheet, a fabric, etc., onto which ink droplets are ejected, and it is used as a generic name of one including a medium to be recorded, a recording medium, a recording sheet, a recording paper, etc.
As a liquid ejection head used as a recording head, a piezoelectric-type head is known which increases a pressure and ejects liquid droplets in such a manner that a vibration plate is displaced with a piezoelectric actuator or the like to change a volume in a liquid chamber. Also, a thermal-type head is known which ejects liquid droplets in such a manner that a pressure in a liquid chamber is increased by air foam generated due to heat by a heating element that is provided in the liquid chamber and generates heat upon energization.
In such an image forming apparatus of a liquid ejection type, an improvement in an image formation throughput, i.e., acceleration of an image formation speed has been particularly demanded. The image forming apparatus supplies ink from a high-capacity ink cartridge (main tank) installed in the main body of the image forming apparatus to a sub-tank (including one called a head tank or a buffer tank) arranged above the recording head via a tube. Supply of the ink via the tube in this manner (tube supply method) makes it possible to reduce the size and weight of a carriage part and greatly reduce the size of the apparatus including a structure system and a driving system.
Meanwhile, in the tube supply method, ink to be consumed from the recording head during image formation is supplied from the ink cartridge to the recording head via the tube. However, if a thin tube excellent in flexibility is used, fluid resistance becomes large when the ink flows in the tube. Therefore, supply of the ink may be delayed at the ejection of the ink, which results in an ejection failure. Particularly, in a large-sized machine that performs printing on a wide recording medium, the length of a tube is necessarily long and the fluid resistance of the tube becomes large. Furthermore, when printing is performed at high speed and when high-viscosity ink is ejected, the fluid resistance becomes large. Therefore, short supply of the ink to the recording head occurs.
In order to address these problems, a method disclosed in Patent Document 1 is known. Specifically, ink in an ink cartridge is maintained in a pressurized state, and a differential pressure regulating valve provided on the upstream side of a head for supplying the ink is used to supply the ink when a negative pressure in a sub-tank becomes greater than a predetermined pressure.
In addition, a method disclosed in Patent Document 2 is known in which ink is fed by a pump to a negative pressure chamber that receives a negative pressure with a spring arranged above a head to positively control a ink supply pressure. Furthermore, a method disclosed in Patent Document 3 is known in which a pump is used to positively control a pressure without a negative chamber.
In addition, a method disclosed in Patent Document 4 is known in which an ink circulation system is connected to a recording head having two ink supply ports to control the pressure of the head in accordance with the flow rate of a circulation pump.
On the other hand, as a method for obtaining a negative pressure with a simple configuration, an ink cartridge in communication with air is connected to a recording head via a tube so that the ink cartridge is arranged beneath the recording head. With this arrangement, a water head difference is generated to obtain the negative pressure.
In spite of its simple configuration, this method can provide a more stable negative pressure than a method for applying a pressure at all times with a negative pressure geared valve or a method for feeding ink with a negative pressure chamber. However, this method using a water head causes a pressure loss due to resistance of a tube as described above.
As a technology for solving this pressure loss in the ink supply system that obtains a negative pressure using a water head, Patent Document 5 discloses a configuration in which a pump is provided in a tube connecting a head to an ink cartridge, a bypass is provided between the upstream side and the downstream side of the pump, and a valve is provided in the bypass. An opening degree of the valve provided in the bypass is appropriately controlled according to printing to maintain a desired pressure.    Patent Document 1: JP-B2-3606282    Patent Document 2: JP-A-2005-342960    Patent Document 3: JP-A-5-504308    Patent Document 4: JP-A-2006-159811    Patent Document 5: JP-A-2004-351845
The technology disclosed in Patent Document 1 solves the problem of the short supply of the ink described above, but its mechanism for controlling a negative pressure is complicated and high sealing performance of a negative pressure geared valve is required. In addition, since a pressure is applied at all times, high airtightness of all connection parts in an ink supplying path is required. In the event of a failure, the ink may be spouted from the ink supplying path.
Furthermore, since a pressure is positively controlled by the pump in the technologies disclosed in Patent Documents 2 through 4, a feeding amount by the pump must be correctly controlled in accordance with a consumption amount of the ink. Therefore, feedback control or the like using the pressure of the negative pressure chamber is required. In addition, when the technologies are applied to an image forming apparatus using, for example, plural different colors of ink, the pump must be controlled for each color of the ink, which results in complicated control of the apparatus and an increased size of the apparatus.
Similarly, when the technology disclosed in Patent Document 5 is applied to the image forming apparatus using plural different colors of ink, the pump must be controlled for each color of the ink, which results in an increased size of the apparatus.